RichEditBox loses formatting for empty text

I have something like a diagramming UWP application where each shape has an RichEditBox. The problem is, that I loose the formatting when the text is empty.

For Example:

1. Change the text to bold: EditBox.Document.Selection.CharacterFormat.Bold = FormatEffect.On.
2. Delete all text
3. Get the text via "EditBox.Document.Selection.GetText(GetTextFormat.FormatRtf, out text);

=> The result is just an empty string, not valid RTF document at all. This is a problem in my scenario. Is there an approach to solve it? I think RichEditBox should always provide a valid RTF document.